This invention relates to a continuous process for solid state polymerization of polyesters in a moving bed reactor process.
High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same compositions (polyester prepolymers) by solid state polymerization of pellets or chips (granules) made in a continuous moving-bed or static bed process (so called because the polymer bed is not fluidized).
In a continuous moving-bed process, polymerization is carried out at temperatures above the prepolymer glass transition temperature but below the melting point of the polyester resin. Polyester prepolymer is fed into the top of a solid state polymerization reactor in which it moves down by gravity in contact with a stream of preheated purge gas. The gas primarily functions to carry off unwanted by-products such as glycols, water and acetaldehyde. Polymerization proceeds as the polyester moves through the reactor. A smooth and continuous flow of polyester granules must be maintained. Agglomeration of polyester granules and sticking of granules to the reactor walls interferes with smooth flow and makes discharge of the granules from the reactor difficult. Therefore, the temperature of the granules must be maintained sufficiently low to prevent agglomeration but high enough to achieve an acceptable polymerization rate.
Since the primary function of the purge gas is to carry off unwanted reaction by-products, its flow rate should be just sufficient to effectively remove the reaction by-products. Excessive -Flow rate results in high costs of supply, regeneration, and disposal.
Small granules or pellets of polyester result in faster polymerization but have a greater tendency to agglomerate. Partially crystallizing the granules helps reduce agglomeration. It is, therefore, important to achieve a proper balance of reaction temperature, velocity of downward movement of polymer granules, prepolymer properties and other process conditions for successful commercial operation.
Conventional static bed solid state polymerization processes use a single reactor for each processing line. A single reactor is satisfactory for producing polyester with an increase of molecular weight by an amount up to that corresponding to an intrinsic viscosity increase of about 0.25 to 0.3 dl/g. They are, however, limited in ability to increase molecular weight because of the inability to achieve sufficient reactor residence time. Growing demand has given rise to a need for a process that can achieve higher molecular weight resins.
I have discovered a process that uses two or more static bed reactors in series to achieve polyester molecular weight increases greatly above those previously possible while avoiding unwanted agglomeration and other undesirable side effects.